[systemd-devel] Creating executable device nodes in /dev?

[Topi Miettinen]

On 8.12.2020 20.07, Andy Lutomirski wrote:
> On Thu, Nov 19, 2020 at 10:05 AM Topi Miettinen <toiwoton at gmail.com> wrote:
>>
>> On 19.11.2020 18.32, Zbigniew Jędrzejewski-Szmek wrote:
>>> On Thu, Nov 19, 2020 at 08:17:08AM -0800, Andy Lutomirski wrote:
>>>> Hi udev people-
>>>>
>>>> The upcoming Linux SGX driver has a device node /dev/sgx.  User code
>>>> opens it, does various setup things, mmaps it, and needs to be able to
>>>> create PROT_EXEC mappings.  This gets quite awkward if /dev is mounted
>>>> noexec.
>>>>
>>>> Can udev arrange to make a device node executable on distros that make
>>>> /dev noexec?  This could be done by bind-mounting from an exec tmpfs.
>>>> Alternatively, the kernel could probably learn to ignore noexec on
>>>> /dev/sgx, but that seems a little bit evil.
>>>
>>> I'd be inclined to simply drop noexec from /dev by default.
>>> We don't do noexec on either /tmp or /dev/shm (because that causes immediate
>>> problems with stuff like Java and cffi). And if you have those two at your
>>> disposal anyway, having noexec on /dev doesn't seem important.
>>
>> I'd propose to not enable exec globally, but if a service needs SGX, it
>> could use something like MountOptions=/dev:exec only in those cases
>> where it's needed. That way it's possible to disallow writable and
>> executable file systems for most services (which typically don't need
>> /tmp or /dev/shm either). Of course the opposite
>> (MountOptions=/dev:noexec) would be also possible, but I'd expect that
>> this would be needed to be used more often.
>>
> 
> I imagine the opposite would be more sensible.  It seems odd to me
> that we would want any SGX-using service to require both special mount
> options and regular ACL permissions.

How common are thes SGX-using services? Will every service start using 
it without any special measures taken on it's behalf, or perhaps only a 
special SGX control tool needs access? What about unprivileged user 
applications, do they ever want to access SGX? Could something like 
Widevine deep in a browser need to talk to SGX in a DRM scheme?

> As  a further argument, I just did this on a Fedora system:
> 
> $ find /dev -perm /ugo+x -a \! -type d -a \! -type l
> 
> No results.  So making /dev noexec doesn't seem to have any benefit.

It's no surprise that there aren't any executables in /dev since 
removing MAKEDEV ages ago. That's not the issue, which is that /dev is a 
writable directory (for UID=0 but no capabilities are needed) and thus a 
potential location for constructing unapproved executables if it is also 
mounted exec (W^X).

-Topi